Lethargic boy has gun trained on one (7)
I believe the answer is:
languid
'lethargic' is the definition.
(similar in meaning)
'boy has gun trained on one' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'has' is an insertion indicator.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'on' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'gun' is an anagram of 'ngu'.
'ngu'+'i'='ngui'
'lad' placed around 'ngui' is 'LANGUID'.
(Other definitions for languid that I've seen before include "Listless (7)" , "Dreamy or lackadaisical" , "Lauding for being listless" , "Sluggish" , "Slow" .)